What visual image information is needed for the things we do? How is vision used to acquire information from the world?

Types of Eye Movement Information GatheringStabilizing Voluntary (attention)Reflexive Saccadesvestibular ocular reflex (vor) new location, high velocity, ballisticbody movements Smooth pursuitoptokinetic nystagmus (okn) object moves, velocity, slow whole field image motion Vergence change point of fixation in depth slow, disjunctive (eyes rotate in opposite directions) (all others are conjunctive) Fixation: period when eye is relatively stationary between saccades.

Catching: Gaze Patterns Catcher Thrower saccade X X smooth pursuit X Terminology: saccadic eye movement

Catching: Gaze Anticipation CatcherThrower X X X 61 ms -53 ms Timing of departure and arrival linked to critical events Saccade reaction time = 200ms

What is the significance of prediction? Brain must learn the way ball moves etc and program movement for an expected state of world. Not reacting simply to current visual information. Stimulus Response What is meant by an “internal model”?

Photoreceptors ganglion cells LGN Primary visual cortex other cortical areas mid-brain brain stem muscles Why is prediction necessary? Analysis of visual signals takes a lot of time! Round trip from eye to brain to muscles takes a minumum of 200 msec. Cricket ball only takes about 600 msec. Prediction gets around the problem of sensory delays.

20 deg Accuracy of Fixations near Bounce 2D elevation Subjects fixate above the bounce point bounce point How good is Internal Model?

Poor tracking when ball is unexpectedly bouncy

Better tracking 2 trials later.

Pursuit accuracy following bounce tennis ball bouncy ball Does pursuit accuracy improve with repeated trials? Does it matter which ball is used first? What can we conclude if it does? 5 subjects Measure proportion of time between bounce & catch that eye is close to ball

Prediction in Squash

These speeds are much higher than expected. Too high for a reactive system. That is, prediction is necessary. 100deg/sec = 10 deg in 100 msec. “Reduced gain” means eye lags behind ball. “Gain” = 1 means perfect tracking

Macular degeneration patient (blind in central 15 deg field) Fovea not necessary for smooth pursuit - can learn to use other regions

Binocular Vision Stereoscopic information: image in the two eyes is different. This information is used to perceive the depth relations in the scene. When is stereoscopic information useful? - reaching and grasping - walking over obstacles - catching?? Development of stereoscopic vision - amblyopia/ astigmatism - critical period

Difference in retinal distance between the objects in the two eyes is called “retinal disparity” and is used to calculate relative depth.

Binocular Vision

Monocular Vision

Other information that may be useful for catching. Motion parallax: change in relative position of objects at different depths when the head moves. Looming: image of ball increases in size as ball gets closer. Rate of change of size can be used to calculate “time-to-contact” Pursuit movement: keeping the eye on the ball.

target selection signals to muscles inhibits SC saccade decision saccade command Cerebellum Learning? Planning?

signals to muscles detect/analyze retinal image motion decision to pursue/attention prediction/ learning? Supplementary eye fields planning?

Even the simplest action must involve linkage between memory, vision, eye movements, and body movements. from Land et al, 1999

1. What are the questions? Is the behavior observed by Land in cricket also true for a simple task like catching a ball? What eye movements are made in this case? Do subjects anticipate the bounce point? By how much? Do Subjects look at floor or above the bounce point? What happens after bounce? How do subjects adjust to different balls? ….. Is there a difference between throwing and catching? Why? What eye movements are made when observing others throw and catch? Similarity between individuals? When do the hands start to move? 2. Choice of task: Catching and throwing a ball. 3. Procedure: Select subject and calibrate eye tracker. Three people stand at equal distances apart and throw the ball back and forth, with a bounce in the trajectory. Need to measure this distance. First throw in a predictable manner, about 8 times. Then use a different ball, 8 trials. Other balls… Compare one versus two eyes???

2.Data analysis Label your tape. Play it frame-by-frame on the VCR in the lab. …. What to look for: –Describe eye movements sequence for each trial eg Trial 1: fixate near hands/saccade to bounce point/fixate/track portion of trajectory/fixate for last part of trajectory (??) Trial 2: fixate near hands/saccade to bounce point/fixate/track portion of trajectory/fixate for last part of trajectory (??) …. B How regular is the sequence of movements? C What is the timing of the saccades/fixations/tracking relative to movement of the ball. How much do subjects anticipate the bounce point, if at all? D. How accurate are fixations near the bounce point? (Need to measure visual angle.) –Compare different conditions. –What happens with the different balls? Do the eye movements change with additional experience? How quickly do they adjust? Other Aspects: –When do hands start to move, relative to release of ball? –How similar are different individuals? Where would we expect similarities/ differences? What is the role of the pursuit movement? If pursuit is made only on final bounce, implies pursuit is used to guide hands. Maybe position of eye in head. Binocular information versus monocular (looming)

Different gaze pattern for watching but still anticipate bounce and catch events.

Catcher Thrower saccade X X Gaze Patterns Different when Watching X

CatcherThrower X X Watching:Gaze Anticipation X -51 ms -167 ms -517 ms Head rotation begins msec before release

Time of departure/arrival relative to: Anticipation Patterns for Catching vs Watching ReleaseBounceCatch Catching 61 msec-53msec -35 msec Watching -51 msec-167 msec-517 msec

Throwing saccade catcher thrower -440 ms -330 ms X X